MySQL - 连接3个表并将一行作为一列

Question

我试图在现有线程上寻求帮助，但我发现自己因为答案而迷失了方向。

这是我的情景：

我有两个表：ACCOUNTS，NET_PROCEEDS

ACCOUNTS 表格有：

1. ACCOUNT_ID
2. ACCOUNT_NAME
3. ACCOUNT_DESCRIPTION

NET_PROCEEDS 表格包含：

1. Net_Proceeds_ID
2. ACCOUNT_ID
3. Fiscal_Year
4. 金额

现在我的基本sql可以使用此输出连接两个表：

Account_ID | Account_Name | Account_Description | Fiscal_Year |量

但我正试图推出这个输出：

Account_ID | Account_Name | Account_Description | 2016 | 2017年| 2018年| 2019

fiscal_year将成为值为amount的列。

有什么想法？我将感谢你的帮助。 由于机密性，我无法共享代码。感谢。

Answer 1

这有两种方法，取决于两个表中的Account_ID是否唯一。 更快的方法（如果Account_ID对每个表都是唯一的）：

SELECT A.*, N16.Amount as Amount_2016 
, N17.Amount as Amount_2017 
, N18.Amount as Amount_2018 
, N19.Amount as Amount_2019 
FROM ACCOUNTS A 
LEFT JOIN NET_PROCEEDS N16 ON N16.Account_ID = A.Account_ID AND N16.Fiscal_Year = '2016'
LEFT JOIN NET_PROCEEDS N17 ON N17.Account_ID = A.Account_ID AND N17.Fiscal_Year = '2017'
LEFT JOIN NET_PROCEEDS N18 ON N18.Account_ID = A.Account_ID AND N18.Fiscal_Year = '2018'
LEFT JOIN NET_PROCEEDS N19 ON N19.Account_ID = A.Account_ID AND N19.Fiscal_Year = '2019'

更安全的方式（如果Account_ID不是/可能不是每个表唯一的）：

SELECT A.*, (SELECT SUM(N16.Amount) FROM NET_PROCEEDS N16 ON N16.Account_ID = A.Account_ID AND N16.Fiscal_Year = '2016') as Amount_2016 
, (SELECT SUM(N17.Amount) FROM NET_PROCEEDS N17 ON N17.Account_ID = A.Account_ID AND N17.Fiscal_Year = '2017') as Amount_2017
, (SELECT SUM(N18.Amount) FROM NET_PROCEEDS N18 ON N18.Account_ID = A.Account_ID AND N18.Fiscal_Year = '2018') as Amount_2018 
, (SELECT SUM(N19.Amount) FROM NET_PROCEEDS N19 ON N19.Account_ID = A.Account_ID AND N19.Fiscal_Year = '2019') as Amount_2019 
FROM ACCOUNTS A 

如果在Account_ID不唯一时使用第一个，它将为Account_ID的每个重复实例创建多行，这将混乱任何SUM或COUNT等。